Bias sensitivity to magnetic fields is an important performance parameter for optical gyroscopes, such as fiber optic, ring laser, and other similar optical gyroscopes used for inertial sensing. External sources of ambient magnetic fields such as the Earth's magnetic field, electrical machinery, etc., can cause bias errors in an optical gyroscope. For example, for a fiber optic gyroscope used in an inertial navigation system, the allowable magnetic bias sensitivity of the instrument is typically between 0.001 and 0.0001 degrees per hour per gauss (deg/hr/gauss). In contrast, the inherent sensitivity of an unshielded gyroscope is on the order of 1 deg/hr/gauss.
An effective method of minimizing bias sensitivity to an external magnetic field is to reduce the magnitude of the local field by the addition of a magnetic shielding structure (“magnetic shield”) around the gyroscope. Magnetic shields are typically made from a high-permeability material that functions as a preferred path for an ambient field. Essentially, the magnetic shield may shunt the ambient magnetic flux around the gyroscope located inside. Thus, the magnetic shield may reduce the effect of an ambient magnetic field on the gyroscope.
Fiber optic gyroscopes may include rotation sensitive optical fiber wrapped into a coil (“sensing coil”) in order to detect motion. However, the bias sensitivity of a sensing coil to an external magnetic field may be orientation dependent. For example, the magnetic sensitivity of a depolarized fiber optic gyroscope may be ten times greater in the direction of the rotation sensing axis (e.g., longitudinal axis of the sensing coil) than in the direction perpendicular to the rotation sensing axis. Thus, it is advantageous for magnetic shielding to be most effective parallel to the direction of the sensing axis if the effect of an ambient magnetic field on the bias sensitivity of the gyroscope is to be significantly reduced.
Currently, magnetic shields are usually manufactured in two pieces for ease of fabrication and assembly. Prior art magnetic shields are typically assembled such that the two pieces are connected at a seam perpendicular to a longitudinal axis of the magnetic shield. Furthermore, fiber optic gyroscopes are usually oriented within the magnetic shield such that the direction of the sensing axis of the fiber optic coil is aligned with the longitudinal axis of the magnetic shield. Due to the orientation of the seam in prior art magnetic shields, this method of assembly creates a discontinuity in a magnetic flux path parallel to the sensing axis, reducing the shielding effectiveness in that direction. As a result, the most sensitive orientation of the depolarized fiber optic gyroscope sensing coil may be parallel to the least effective orientation of the magnetic shield.
Accordingly, it is desirable to have a magnetic shield for a fiber optic gyroscope with an improved mating design that overcomes the above deficiencies associated with the prior art.